Cytopathology provides diagnostic evaluation of cytology specimens that direct patient management and treatment. The Cytopathology Section provides complete diagnostic service in cytopathology for the various clinical protocols at the NIH. We specialize in the application of ancillary techniques (i.e., immunoperoxidase, flow cytometry, and, most recently, molecular testing) to patient material for the confirmation of morphologic diagnoses,evaluation for protocol entry criteria and collaborative investigations. The Cytopathology Section has an approximate distribution of specimens as follows: fine needle aspiration (FNA), 18 percent; central nervous system, 35 percent; effusions, 6 percent; respiratory tract, 8 percent; genitourinary tract, 21 percent; cervical/vaginal, 8 percent; miscellaneous, 4 percent. Due to the nature of the specimen material evaluated by our Section, often of our cases require immunoperoxidase studies (11 percent). The immunosuppressed nature of our patient population at the NIH dictates that a significant proportion of our cases require special studies for pathologic organisms (11 percent). The relatively high rate of pathologic findings combined with the diversity of types of exfoliative and FNA specimens provide a broad experience in diagnostic cytopathology for residency and fellowship training. The Cytopathology Section is involved in numerous clinically related research studies, many of which utilize FNA or exfoliative samples with immunocytochemistry and/or molecular techniques to provide ancillary diagnostic information. A partial listing of such studies includes: (1) evaluation of expression of malignant melanoma markers (MART-1 and gp100) through the utilization of antibodies in ex-vivo FNAs from malignant melanoma patients; (2) evaluation of expression of epithelial markers (CK AE1/AE3 and CK 8/18) through the utilization of antibodies in ex-vivo FNAs from gastrointestinal carcinoma patients; (3) evaluation of expression of epithelial markers (CK AE1/AE3 and CK 8/18) through the utilization of antibodies in ex-vivo FNAs from non-gastrointestinal carcinoma patients; (4) morphologic and immunocytochemical evaluation of tumor infiltrating lymphocytes samples for possible tumor cell contamination prior to therapy; (5) FNA material for subsequent analysis by polymerase chain reaction (PCR), microarray, and other molecular technologies; (6) evaluation of cell lines by morphology and immunocytochemistry; and (7) application of RNA-based molecular technique to non-gynecologic and FNA cytology samples to enhance diagnosis, prognosis and patient management.